Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available.
A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic liquid crystals, one class of which are termed ferroelectric liquid crystals, in devices which give microsecond switching and bistable operation. Ferroelectric liquid crystals were discovered by R. B. Meyer et al. (J. Physique 36, 1-69 (1975).). A high speed optical switching phenomenon using a "surface-stabilized ferroelectric liquid crystal" (SSFLC) was discovered for the ferroelectric liquid crystals by N. A. Clark et al. (Appl. Phys. Lett. 36, 899 and U.S. Pat. No. 4,367,924).
Many new ferroelectric liquid crystals have been developed and their switching characteristics extensively tested. Although devices employing these materials exhibit high response speed and wide viewing angles, many problems remain in developing SSFLC devices. These problems have included insufficient threshold characteristics, unsatisfactory contrast (due to chevron defects), and insufficient bistability due to difficulties in controlling alignment.
More recently, antiferroelectric liquid crystals (AFLC), another class of tilted chiral smectic liquid crystals, have been developed. Antiferroelectric liquid crystals are switchable in a chiral smectic C.sub.A phase (SC.sub.A * phase) in addition to the tilted chiral smectic C phase (S.sub.C * phase) used in ferroelectric liquid crystal devices.
Devices employing antiferroelectric liquid crystals have been described by Chandani et. al (Japan J. of Applied Physics 27(5), L729-732 (1988).). Antiferroelectric liquid crystals used in these devices exhibit three stable states: two stable states under the influence of an electric field and a third antiferroelectric state in the absence of an electric field. Antiferroelectric liquid crystals are characterized by having a distinct threshold and a double hysteresis that allows for a memory effect in either of the driven states. Antiferroelectric liquid crystals can be easily switched and provide devices that have few defects and that allow for the recovery of alignment.
In an AFLC device, with no applied electric field, an AFLC composition has a layered structure comprising many smectic layers, with the molecules of each layer being tilted in a direction opposite to those of the adjacent layer such that the liquid crystal composition has no net polarization. The alternating molecular director also results in a uniform optical axis parallel to the layer normal of the smectic layers. When placed between a pair of crossed polarizers such that one of the polarization axes of the polarizers is aligned with the uniform optical axis of the composition, the device exhibits a dark state. When an electric field is applied, the liquid crystal orients to align the spontaneous polarization with the electric field, resulting in one of two bright states, depending on the polarity of the electric field. Tristable switching behavior has also been observed for twisted ferroelectric and deformed helix devices.
Although AFLC devices of the prior art have provided tristable switching, there remains a need in the art for liquid crystal display devices that can provide tristable switching, gradation display (grey scale), threshold control, hysteresis control, and fast response times, and that can be used in both small and large size displays. In addition, there remains a need in the art for liquid crystal display devices that overcome the limitations of the prior art as to polarization, threshold voltage control, and contrast.